Funnily enough I had this conversation yesterday. Definitely knees as without elbows we wouldn't be able to do half the things we can now, such as play most games, musical instruments, EAT :p ect ect.
Knees are just for walking and running, and as fun as that is, I would rather eat :p Plus we could just develop strap in segways (sp?) which could replace walking
Right, would you rather be living in present times, or 1000 years ago?